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5E3 split load PI to 6G3 longtail PI ? preamp ?

4/4/2005 4:30 PM
William Jahnkow
5E3 split load PI to 6G3 longtail PI ? preamp ?
Here is what I want to do ;  
Instead of the split load PI I want to go to a long tail such as on the 6G3 DELUXE .  
From what I understand a split down is a PI with the gain stage as part of the PI . ( stop me if I'm wrong )  
If I do this I want to use the second half of V1 as the gain and use a longtail PI .  
I notice one difference on the cathode bias resister and the plate load resister .  
On using one half of V1 the cathode resister is normally 1500 ohm , using both as on 5E3 it is 820 ohm . This is using a split olad PI .  
One the 6G3 deluxe on the preamp stage of V1 which is using both halfs for the two volume pots and jack pairs it is 1500 ohms so here I would think to use 3000 ohm for one half . it seems as if you do not want to drive the preamp as hard as with a split load PI . And you have 220k instead of 100k plate load resisters .  
They are raising both the plate load and cathode resister values here to use a longtail PI .  
then you place the tone and vol pots between the preamp tube and the gain half . The gain stage half is biased just as the v1 preamp of the stock 5E3 .  
then on to the PI .  
I want to do this only if it's a worthwhile change .  
I want to add a treble and bass control in the tone stack . this can be in the same place as the stack is with either type of PI .  
I have the added unused vol pot now since I changed my clone 5E3 to a single vol pot with the pots wired as later fender amps .  
I really would like to know if there is any advantage to a longtail PI over a split load and how I should bias the preamp using the one 12AT7 for both the preamp and gain stage if I go to a longtail PI .  
I hope this make sense and I have been clear . ] the tone stack I have in mind is off a deluxe AA763 BF . This a a little more complicated in the PI because of the tremolo added into the mix .  
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4/5/2005 6:50 AM

The advantage of the long tail phase inverter is it works better with the 6G3's Tremolo, has better rejection of B+ noise and can supply more drive to the output tubes. The split load PI with a gain stage actually has slightly more gain overall.  
The 1500 ohm cathode resistor of the 6G3's V1 stage is used because of the 220K plate resistors. This gives the correct bias to set the plate voltage to about 60% of the B+. If you want to modify the circuit to use 12AT7's, the easiest thing to do is try different resistors until you find the one that gives the correct plate voltage. This is called the "Cut and Try It" method, used by engineers everywhere.  
In a preamp stage, if you change the B+ or the load resistor (or the tube type), many times you must adjust the cathode resistor to bring the plate voltage back to about 50% to 60% of the B+ voltage. Sometimes the plate voltage is adjusted outside this range to generate more distortion. As an example, with a 12AX7, 150K plate resistor and 300V B+, you might end up with a 2K cathode resistor to get the plate voltage to 150V.  
If you install the AA763 tone stack, your amp won't sound like a 5E3 anymore. The tweed character of the 5E3 will be lost and it will be more like the AA964 Princeton. Try it, it may be just what you are looking for.
4/5/2005 12:11 PM
William Jahnkow

I think I understand what you are saying .  
I am still a bit confused about the difference between split load and longtail phase inverters .  
To clarify , does the longtail also have a gain or is the gain provided by the second half of the preamp tube ?  
I know with the 5E3 the preamp is just that a preamp and the gain is provided by the split load PI .  
Right now I use a 12AT7 as the preamp and the split load PI which is all as a 5E3 circuit with the exception that I use one vol pot and this has been wired with the vol pot wiper to the PI tube a 12AX7 . I reversed the leads on the vol pot to make for a more adjustable vol control . I did not like the full on effect right off the bat .  
Using an 12AT7 was done because I was working not with a kit but with the fender schematics that call for a 12AY7 which has even less gain than a 12AT7 .  
I have not checked the 12AT7's plate voltage but have replaced the 820 ohm cathode resister with a 1500 ohm , did'nt notice any change , I did it because I realized I was now only using half the triode and did not want the tube over biased .  
At this point my thoughts are to use the unused half of V1 as the gain stage and place a vol pot between the preamp and gain , V1A and V1B respectivly . and if the longtail PI does not provide gain then I would need to use the V1B for this . Or it may be possible to use the split load PI as is along with the V1B gain stage .  
This is the part I am uncertain of . I do know that if I place a vol pot between V1A and V1B it will need to be used to have any vol or gain provided to the rest of the amp . After this V1B and before the PI I would place the second vol pot and tone .  
As it is now I like the way the amp sounds , it is very clean and detailed . I do loose highs if I back off the guitar vol at all to control the overall volume . I do have to do this because even with the amp at 3 with the guitar on 10 with the best highs overall it is too loud . And I do not have any overdrive until the amp is cranked to 8 but here we are talking real loud .  
This brings to mind the AA763 tone stack in hopes of more treble retention with the guitar cut back , less treble bleed if you will . 250 pf cap verses 500 pf and a bass control that can roll back the lows . Just more semmingly more adjustable .  
So HELP , I don't know which way to go here .  
You see where I'm going , I like the 5E3 but want more control even if it does not react like a 5E3 control wise , I am already out of the 5E3 control setup as it is and I am closer to where I want to be .  
4/5/2005 12:19 PM
Dave H

[QUOTE]Trouble is mine does not seem to have much if any breakup until it is almost wide open .  
Not that I don't like clean full tone , I prefer it but I would like to dial in a bit of breakup if I wanted it .[/QUOTE]  
I thought I’d answered that already in the “where’s the grit” thread. :)  
The 5E3 has one of the best overdrive sounds there is but it’s all about output stage distortion. You can’t get 'that' sound at low volume. It would be a shame to mod the amp into something else without first experiencing the great 5E3 OD. I think you need to change the 12AT7 to a 12AX7 to push the output stage harder then all you need to do is to crank the amp’s volume to about ¾ and the amp will go from clean to OD by using the guitar vol pot. If you’re Strat is muddy with its vol pot below 10 connect a resistor and cap in parallel between the cw and wiper on the guitar vol pot. I think I used 200k and 330p.  
4/5/2005 7:42 PM

Where's that guy who claimed he could make a PI with a gain of 1000 using a dual triode?  
The long tail PI will have almost as much gain as the split load with a gain stage in front of it. The late breakup you are experiencing may be due to the audio taper of your volume control. Don't be concerned unless you don't have enough when it's at max.  
The One option that hasn't been mentioned is running both halves of V1 in parallel like in some Matchless amps. It'll give you a little more gain and a little lower noise.  
For a bass cut control try this: Break the connection between the coupling cap (0.1uF?) and the volume control and insert a parallel combination of a 1000pF cap (maybe 470pf) and a 1meg pot wired as a rheostat. Leave the cap from the tone control connected to the 0.1uF. This network could also go between the second gain stage and the split load if you have the Paul C mod. The stock PI has too high an input impedance for it to work well.

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